1. Field of the Invention
This invention relates to workpiece guides particularly for use in machine tools such as grinding machines in which the elongate rotary workpiece is supported at one or both ends and/or at an intermediate position of its length for a machining operation.
2. Description of the Prior Art
Presently most cam grinding machines utilise workpiece steady rests which have contact shoes which are either set at fixed positions set to support a main bearing at the maximum outer diameter tolerance in which case smaller diameter bearings are loose in the steady rest while camshafts having over tolerance main bearings are marked due to excessive gripping pressure, or two fixed tips may be provided with one spring loaded grip to ensure a clearance free support. However, as main journals may have diametral tolerances of 0.015 mm to 0.02 mm, whilst maintaining shake free support of the journal, the journal centre-line will vary by plus or minus 0.01 mm relative to the axis about which the cams are ground. Thus although the problem of excessive gripping pressure which can arise with a case of three fixed support points is overcome, another problem, that is supporting the journal off-centre from the correct centre-line is introduced.
U.S. Pat. No. 4,276,723 discloses a stead rest for supporting a workpiece to be ground comprising three contact shoes which are simultaneously movable towards and away from the workpiece centre-line so that workpieces of varying diameter can be supported and maintained on a fixed centre line of rotation. The top contact shoe is mounted for pivotal movement to a position clear of the work area to facilitate loading and unloading of the workpiece, one hydraulic operator being provided for pivoting the top contact shoe into and out of its operative position and a second operator being provided for moving the two lower non-pivoting contact shoes towards or away from a workpiece. Whilst provision is made for adjusting the three point support to suit bearings of different diameters, no provision is made for ensuring that the supports hold the bearing on the required centre-line axis whilst at the same time avoiding excessive gripping pressure applied to the bearing.